Radioactive nuclear sources are currently used in industry in a variety of places, including on-line elemental analysis of mining, coal, and cement feedstocks, and sub-surface scanning (e.g. soil composition analysis and landmine detection). The traditional neutron source has been a radioisotope such as 252Cf or Am-Be. Radioisotopes are always on, require shielding, limit types of analysis (e.g. no pulsing or time-of-flight), and pose a personnel hazard during manufacturing and assembly, as well as a security hazard due to threats of so-called “dirty bombs”. Neutrons can also be generated with conventional accelerator technology but these systems have large size and power consumption requirements. Having a compact and efficient fusion neutron generator (FNG) would directly benefit many industries by solving the problems associated with radioactive isotopes while avoiding the complications of large accelerators.
The basic layout of a modern compact accelerator neutron source is shown in FIG. 1. The standard hardware consists of: a high-voltage generator 1 (˜100 kV), a metal hydride target material 2 (usually titanium), one or more accelerator grids 3, an ion source assembly 4 (Penning or RF) and a gas-control reservoir 5 that often uses a hydrogen getter. Operation proceeds as follows: either pure deuterium (D-D system) or a deuterium-tritium (D-T system) mix of gas (up to 10 Ci of T) is introduced into the system at pressures around 10 mTorr; a plasma is generated to provide ions that are extracted out of the source region and accelerated to ˜100 keV; these ions bombard the target 2 where they can undergo fusion reactions with other hydrogen isotopes embedded in the target 2. DD fusion reactions generate 2.45 MeV neutrons and the DT reaction makes 14 MeV neutrons. Exemplary systems can be operated continuously or in pulsed operation for time-of-flight measurements.
There are several major suppliers of non-radioactive neutron generators, all using accelerator-target configurations. List prices range between $85-350K with the highest cost components being the high-voltage power supply, electrical feeds, and interconnects. Lifetime is typically limited by the degradation of the target material and the coating of insulators with best suppliers reporting ˜1000 hours for nominal output levels of 1×106 DD n/s and 1×108 DT n/s, and replacement target units range from $5-50K each. Currently, no suppliers have cost-effective high output (>1E8 n/s) DD systems.
Neutron generators for industrial radioisotope replacement often use the DD fusion reaction because the 2.45-MeV DD neutrons are more easily applied to existing applications that use Cf252, which has an average neutron energy of 2.1 MeV. On the basis of fusion cross section and reaction branching alone, a DT generator has a neutron production rate ˜100 times that of a DD generator, however, the shielding and moderation requirements for 14.1-MeV DT-generated neutrons compared to 2.45-MeV neutrons are much more severe, making DD more attractive for many market applications.